1. Field of the Invention
This invention relates to ink jet inks for use in ink jet printers. More particularly, this invention relates to phase change, or hot melt, ink jet inks for use in ink jet printers from which ink is propelled from the printer nozzle by heat or by a pressure wave.
2. Description of Related Art (Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98)
Ink jet printing involves the placement in response to a digital signal of small drops of a fluid ink onto a surface to form an image without physical contact between the printing device and the surface. The method of drop generation varies among the different ink jet technologies and can be used to classify ink jet printing into two major technology types, continuous (CIJ) and drop-on-demand (DOD).
In CIJ printing systems, a continuous stream of liquid ink droplets is ejected from a nozzle and is directed, with the assistance of an electrostatic charging device in close proximity to the print head, either to a substrate to form a printed image or to a recirculating system. Inks for CIJ printing systems are typically based on solvents such as methyl ethyl ketone and ethanol.
In DOD ink jet printing systems, liquid ink droplets are propelled from a nozzle by heat (thermal or bubble ink jet) or by a pressure wave (piezo ink jet). Unlike CIJ systems, all the ink droplets are used to form the printed image and are ejected when needed, "on demand." No deflection of ink droplets is involved. Thermal or bubble ink jet inks typically are based on water and glycols. Piezo ink jet systems generally use aqueous, solvent, or solid inks. These last inks, also known as phase change inks, are solid at ambient temperature and liquid at printing temperatures. It is these inks to which the present invention relates.
The following properties are required of an ink composition for ink jet printing:
(a) high quality printing (edge acuity and optical density) of text and graphics on substrates, in particular, on uncoated cellulosic paper,
(b) short dry time of the ink on a substrate and good adhesion such that after printing the print is not smudged when rubbed or offset onto a subsequent printed image placed upon the print,
(c) good jetting properties exhibited by a lack of deviation of ink droplets from the flight path (misplaced dots) and of ink starvation during conditions of high ink demand (missing dots),
(d) resistance of the ink after drying on a substrate to water and to solvents,
(e) long-term storage stability (no pigment settling) and
(f) long-term reliability (no corrosion or nozzle clogging).
Inks are known that possess one or more of the above listed properties. However, few inks are known that possess all of the above listed properties. Often, the inclusion of an ink component meant to satisfy one of the above requirements can prevent another requirement from being met. For example, the inclusion of a polymer in the ink composition can improve the adhesion of the ink to the substrate. However, the polymer can impair the jetting of the ink because of the behavior of the polymer under the high shear conditions of jetting (10.sup.4 -10.sup.6 sec.sup.-3). Thus, most commercial ink jet inks represent a compromise in an attempt to achieve at least an adequate response in meeting all of the above listed requirements.
The use of polymers in phase change (hot melt) inks and the printing of such inks has been disclosed in the following publications:
U.S. Pat. No. 5,006,170 teaches the use of "rosin esters" with a colorant and a propellant.
U.S. Pat. No. 5,531,819 teaches the use of an "acrylic resin," "rosin resin," "hydrogenated rosin resin," "petroleum resin," "hydrogenated petroleum resin," or "terpene resin" with a wax, a colorant, and a plasticizer.
U.S. Pat. No. 5,354,368 discloses the use of a "tall oil rosin having a high acid number" with a rheology modifier. Given as examples of "tall oil rosin" are "natural tall oil and wood rosins as well as modified tall oil and wood rosins and tall oil and wood rosin derivatives."
U.S. Pat. No. 5,397,388 teaches the use of "acrylic resin," "rosin resin," "petroleum resin," "modified petroleum resin," "hydrogenated petroleum resin," or "terpene resin," with a wax, an organic substance miscible with the wax, and a colorant.
U.S. Pat. No. 5,409,530 teaches the use of a resin selected from the group consisting of "rosins, rosin derivatives, terpenes, [or] modified terpenes . . . " with a second component to dissolve the resin.
U.S. Pat. No. 5,514,209 discloses the use of a "glycerol ester of a hydrogenated rosin" with a microcrystalline wax.
U.S. Pat. No. 5,620,508 teaches the use of "rosin-type resins" with pigments and oil-soluble dyes. Given as examples of rosin-type resins are "rosins, hydrogenated rosins, disproportionated rosins, rosin esters, rosin-modified phenolic resins, rosin-modified maleic acid resins, and rosin-modified xylene resins.
Also, EP 0 739 958 A2 teaches the use of an "acrylic resin," rosin resin," "petroleum resin," "modified petroleum resin," "hydrogenated petroleum resin," or "terpene resin," with at least one wax, a colorant, and a second resin. One resin has a softening point from room temperature to 100.degree. C., and the other resin has a softening point from 50-150.degree. C.
While the prior art teaches the use of many classes of polymers in phase change (hot melt) inks, no report has been made of the use of crosslinked polymers that decrease in viscosity with increasing shear rate in phase change inks. The essential component of the present invention is a crosslinked polymer that is crosslinked in such a fashion that the links are broken under shear but are reformed at rest. The novel aspect of using a crosslinked polymer in a phase change (hot melt) ink is that the viscosity of the ink will decrease during the jetting operation but then increase when placed upon the substrate. Thus, the jetting properties are improved without diminution of the printing properties. In accordance with the present invention, the preparation of phase change (hot melt) inks useful in ink jet printing devices is provided. The phase change (hot melt) inks preferably are for use in a piezo ink jet printer with which recording is conducted by thermally melting an ink composition at a temperature above ambient temperature (20.degree. C.) and then ejecting the ink composition onto a porous or non-porous substrate such as paper, aluminum, glass, metal, wood, synthetic polymer films, and textiles. Furthermore, the present invention also provides methods for the preparation of crosslinked polymers and for their use in the above-described inks.
The present invention overcomes many of the problems associated with the use of prior art phase change (hot melt) ink compositions while achieving distinct advantages thereof. Accordingly, an object of the present invention is to provide improved ink compositions capable of satisfying simultaneously the properties required of an ink composition for ink jet printing, especially the aforementioned properties (a) to (f) and which comprise a reversibly-crosslinked-polymer that prevents the corrosion to metal components of an ink jet printer often caused by acid-functionalized or acidic polymers in a hot melt ink jet ink. Other objects and advantages of the present invention will become apparent from the following disclosure.